Synchronous machines or synchronous motors can be employed in the drive systems of a passenger transportation apparatus, for example an elevator system, an escalator or a travelator. A synchronous machine has a stator (primary part) and a rotor (secondary part). The stator or primary part can incorporate a stator winding comprised of a plurality of phase windings. A stator current vector with a specific stator current vector direction can be imposed upon the stator winding. A stator current vector of this type is characteristic of the energization of the stator winding. The rotor or secondary part can be configured as an excitation winding or as a permanent magnet. For example, a synchronous machine of this type can be configured as a rotary motor or as a linear motor.
In order to permit the operation of a synchronous machine at optimum efficiency, and to generate the maximum possible effective driving torque on the rotor, an in-phase stator current vector must be imposed upon the stator winding. A corresponding stator current vector with an optimum stator current vector direction is thus imposed upon the stator winding. This optimum stator current vector direction is dependent upon the relative orientation of the rotor to the stator.
This is of particular significance in a passenger transportation apparatus which is driven by linear motors. For example, in an elevator system, the release of the brakes must be exactly synchronized with the application by the linear motor of a driving torque which at least corresponds to the gravitational weight of the—potentially fully-occupied—elevator cage, in order to protect the elevator cage against any uncomfortable subsidence, or even falling.
Specifically upon the starting or start-up of the synchronous machine, the relative orientation of the stator and rotor is generally not known. Although this orientation can be determined by means of sensors such as, for example, incremental transducers, a number of electrical rotations or a certain movement of the rotor are required for this in many cases, as the sensor is generally required to detect a specific reference signal in the first instance. For example, an incremental transducer is firstly required to detect an initial specific reference marker.
Until this reference signal is detected, and the orientation of the stator and rotor can be determined by means of the sensor, the optimum stator current vector direction is generally determined by other methods, for example by means of “test excitations”.
In many cases, however, test excitations of this type are unsuitable for application in a synchronous machine in a passenger transportation apparatus. In general, these test excitations can only operate reliably if a sufficient degree of movement of the of the rotor is possible.
In general, however, for example in elevator systems, this is not the case, specifically upon the starting or start-up of the synchronous machine. For example, if a brake (holding brake) of the elevator system is activated, any movement of the synchronous machine is barely possible, if at all. Moreover, in elevator systems, it can be the case that only a very small amount of mechanical play is available between the rotor of the synchronous machine and a load (cabin or counterweight). Thus, during test excitations, only a severely limited movement of the rotor may be possible. By means of test excitations of this type, it is generally not possible, in an elevator system, to reliably determine an optimum stator current vector direction.
In DE 196 04 701 C1 it is proposed, for the determination of the initial rotor position, that test excitations are executed and the effects thereof upon the rotor are detected and evaluated by means of power or torque sensors. This might be appropriate for a rotary drive; in a linear drive, corresponding sensors for this purpose are required over the entire length of the drive (in a passenger transportation apparatus, a length of several hundred meters in some cases), thereby resulting in high costs.
It is therefore desirable to propose an improved option for the reliable determination of a stator current vector for starting a synchronous machine of a drive of a passenger transportation apparatus.